The Great Belt is 60 km (37 miles) long and 16–32 km (10–20 miles) wide. It flows around two major islands: Samsø in the north and Langeland to the south. At Sprogø the Great Belt divides into the East Channel and the West Channel. Both are traversed by the Great Belt Bridge, but a tunnel also runs under the East Channel.

In pre-glacial times a river, which the Baltic Sea basin then contained and which geologists call the Eridanos, must have passed near the region as the rise of South Swedish Dome in Neogene times diverted it south from its previous path across central Sweden.[1]

The Great Belt originated as Dana River that was eroded into existence 9000–8900 years B.P. when post-glacial rebound made the Ancylus Lake that occupied the Baltic depression lose its outlets around Gothenburg tipping over in the south.[2] The forming of Dana River is thought to have caused a dramatic erosion of sediments, peatlands and forests along its way. This led initially to a relatively rapid fall in the lake level over hundreds of years to then continue falling at a lower pace.[2] Rising sea levels allowed the sea to break through the Dana River forming the Great Belt as a proper seaway. In the processes the Ancylus Lake became the Littorina Sea as salt water entered the Baltic depression.[2]

The Great Belt was historically navigable to ocean-going vessels. It still is used, despite a few collisions and near collisions with the Great Belt Bridge. The Danish navy monitors maritime traffic in the waters around the Great Belt.

In the reign of king Eric of Pomerania the Danish government began to receive a large part of its income from the so-called Sound Dues toll on international merchant ships passing through the Øresund. Non-Danish vessels were restricted to the Øresund channel. Merchants paid the tax under threat of having their vessels sunk or confiscated.

During the middle of the 19th century, this practice became a diplomatic liability and the Danish government agreed to terminate it, achieving an international financial compensation in return. Danish waterways were consequently opened to foreign shipping. The eastern half of the Great Belt is an international waterway, legally based on the 1857 Copenhagen Convention.[4] The western half of the Great Belt (between Funen and Sprogø) and all other parts of the Danish straits are Danish territorial waters and subject to Danish jurisdiction.[citation needed]